1. Field of the Invention
The present invention relates generally to techniques in transmission power control of base stations in a CDMA (code division multiple access) cellular system using spread spectrum techniques. More specifically, the present invention relates to a power control method on forward links (viz., base station to mobile unit links) in a CDMA cellular system in order to increase capacity of the overall system.
2. Description of the Related Art
As is well known in the art, in a CDMA system, all users transmit simultaneously and at the same frequency. The transmitted signals occupy the entire system bandwidth, and code sequences, which are orthogonal, are used to separate one user from another. That is, each user is assigned a unique code sequence. The use of the same frequency in the overall system indicates that no "handoff" from one frequency to another is needed as in FDMA (frequency division multiple access) and TDMA (time division multiple access) systems. This is called a soft handoff that is disclosed in U.S. Pat. No. 5,101,501 by way of example.
In a CDMA system, there is no distinct limit on the number of users. The system performance for all users degrades gradually as the number of active users increases. More specifically, mobile units in the CDMA system transmit independently (viz., asynchronously) from each other. This means that their signals arrive randomly at the base station and therefore, the crosscorrelation between these randomly arrived signals is not zero and thus causes interference.
The major difficulty with CDMA is a so-called "near-far effect", which occurs when a weak signal received at the base station from a distant mobile unit is overpowered by a strong signal from a nearby interferer. To reduce the near-far effect, power control on reverse links (viz., mobile unit to base station links) is necessary.
Additionally, the system capacity is expanded by power control on the forward links (viz., base station to mobile unit links). One example of such power control on the forward link is disclosed in Japanese Laid-open Patent Application No. 7-38496. According to this conventional technique, each of the mobile units in a given cell receives a pilot signal from the cell's base station, measures a signal-to-noise (S/N) ratio using the pilot signal received, and then informs the base station of the measurement results. The base station responds to the measurement results and controls the transmission power on the forward link of each mobile unit. Thus, the S/N ratios at the mobile units within the cell are improved and approach a predetermined level (viz., roughly equalized). As a result, a low level of interference is achieved at each mobile unit.
This conventional technique, however, has suffered from a drawback. That is, when a S/N ratio at a given mobile unit is lowered due to increase in the number of the active users in the cell, the base station is responsive to the reduced S/N ratio and raises the power on the forward link to the given mobile unit. This in turn undesirably lowers the S/N ratio at each of other mobile units, with the result that the S/N ratio of the first base station again is lowered. This cycle is repeated and eventually the power of each forward link of many mobile units undesirably is raised to the maximum value.
Further, it takes a relatively long time until the lowering of interference is carried out after the measurement of the S/N ratio. Therefore, during the long feedback time, the S/N ratio measured has undesirably changed. In such a case, a precise control is no longer expected.